1. Field of the Invention
The present invention relates to an ink jet recording apparatus and method of the thermal type and, more particularly, to an ink jet recording apparatus and method using a plurality of heat generating elements disposed inside each of nozzles for ejecting ink.
2. Related Background Art
Various recording devices are used under various circumstances nowadays and, particularly, the ink jet recording apparatus of the thermal type is widespread as a recording apparatus quiet in operation and simple in structure. This ink jet recording apparatus of the thermal type is constructed generally such that a lot of nozzles for ejecting the ink from a recording head are juxtaposed and that a heat generating element is disposed inside each nozzle. In the ink jet recording apparatus of this type, a bubble is generated momentarily in the ink by heat generated by each heat generating element and the ink is ejected from the recording head by pressure of the bubble.
There is an example of such ink jet recording apparatus proposed in such an arrangement that a plurality of (for example, two) heat generating elements are disposed inside each nozzle and that the number of heat generating elements driven at one time is changed to change an ejection amount of ink, thereby permitting two-stage switching of resolution. Specifically, the two heat generating elements in each nozzle are driven simultaneously for carrying out recording in a low resolution but at high speed, whereby a large quantity of the ink is ejected from each nozzle to record a large dot on a print sheet. Only one of the heat generating elements is driven in each nozzle for carrying out recording in a high resolution but at low speed, whereby the ejection amount of ink is reduced to a half to record a small dot on the print sheet.
Also described is a configuration for realizing recording in multi-level gradations by the similar arrangement in which a plurality of heat generating elements are disposed inside each nozzle and in which the ejection amount of ink is changed according to the number of heat generating elements driven at one time.
There is also another proposal for increasing the ejection efficiency of ink by designing of pulses applied to the heat generating elements. Namely, it is found that the ejection amount of ink is increased by application of a divided pulse rather than by application of a simple single pulse to the heat generating element, even though the energy applied is equal. There is thus such a proposal that two heat generating elements are disposed inside each nozzle, a divided pulse is applied to one heat generating element for recording of a small dot, and a divided pulse is applied to the both of the two heat generating elements for recording of a large dot, thereby recording small and large dots with high efficiency.
With the ink jet recording apparatus wherein the plural heat generating elements are disposed in each nozzle as described above, gradients of image recording can be changed in dot units.
For changing gradients in two levels as described above, a preferred and simple structure is such that only two heat generating elements having the same size and shape are provided in each nozzle. It was, however, found that it was difficult to double the ejection amount of ink by changing the number of heat generating elements driven between one and two in this structure. Namely, it was difficult to generate a difference enough to achieve the two-level gradients, because the diameter of the small dot was not small enough as compared to the diameter of the large dot.
It is not impossible to generate a sufficient difference between the small dot and the large dot, for example, by disposing three or more heat generating elements inside each nozzle, driving only one heat generating element for formation of a small dot, and driving all the heat generating elements for formation of a large dot. Further, it is not impossible to generate a sufficient difference between the small dot and the large dot by forming a small heat generating element and a large heat generating element in each nozzle, driving only the small heat generating element for formation of a small dot, and driving all the heat generating elements for formation of a large dot.
However, these techniques decrease the productivity of the ink jet recording apparatus, because the structure of the heat generating elements is complicated and the designing of the heat generating elements becomes more difficult. In addition, the resolution also decreases, because it becomes difficult to locate the nozzles in high density. It is thus preferred that the small dots and large dots be able to be recorded with a sufficient difference by the simple structure in which only two heat generating elements of an identical shape are disposed in each nozzle.
The present invention has been accomplished in view of the above issues and an object of the present invention is to provide an ink jet recording apparatus and method capable of generating the sufficient difference for multi-level resolutions by simple structure.